1. Field of the Invention
The present invention relates to a method of operating a communications system and particularly, but not exclusively, to a method of handover in a frequency division multiplex (FDM)-time division duplex (TDD) communications system.
The present invention also relates to a communications system for operating in accordance with the method and to a secondary station for use in the system.
2. Related Art
A known FDM-TDD cordless telephony system is known in Great Britain as CT2 and the common air interface is disclosed in MPT 1375 "Common Air Interface Specification" to be used for the interworking between cordless telephone apparatus including public access services, published by the Department of Trade and Industry, London. The CT2 system comprises a plurality of fixed, geographically separate primary (or base) stations which are connected by landline links to interface with the public switched telephone network (PSTN). The primary stations form a plurality of zones which for CT2 are not necessarily contiguous. Each primary station has at least one transceiving means capable of communicating by any one of a plurality of FDM-TDD channels with a secondary station which is able to roam within its cell. When a secondary station is sufficiently far from its primary station, the interference on the existing FDM-TDD channel becomes unacceptable, handover to another channel or to another, in range, primary station becomes necessary if the call is to continue. Under the provisions of Section 17.5 of MPT 1375, a secondary station must wait at least three seconds before it is able to initiate a handover routine. This routine requires the secondary station to monitor all the other FDM channels in the system noting their quality and then requesting the primary station to effect handover to a channel which is considered to be acceptable. As part of the handover routine, information including the addresses of the respective parties participating in a call and authorisations may be communicated. The handover routine incurs a time delay penalty which, if it results in there being no valid handshake exchanged within ten seconds between the primary station and secondary station, then the call will be curtailed and the user cut off.